A Rain Fall Warning has been issued for Brockville – Leeds and Grenville, Smiths Falls – Lanark County, Cornwall – Morrisburg and City of Ottawa.

An area of low pressure over the Great Lakes and one coming up from the U.S. Eastern Seaboard will combine to form a powerful nor’easter that will tap into tropical moisture, bringing heavy rain and strong winds to eastern Ontario, southern Quebec, and the Maritimes.

Rain from the Great Lakes system is expected to fall in parts of southern Quebec and eastern Ontario late Saturday, but will likely pick up in intensity as the east coast storm moves in Sunday through Monday.

Given the abundant moisture that will accompany the storm, localized areas between eastern Ontario and southwestern Quebec could pick up 100+ mm of rain.

“The greatest uncertainty is on the location of the back edge of the storm as there will be a sharp cut-off along the western side of the storm, with very heavy rain for parts of eastern Ontario and little rain just to the west of those areas,” says The Weather Network’s Dr. Doug Gillham.

The intense rains raise the risk of localized flooding, but the system’s strong winds will also be a major threat.

Gusts of 60-80 km/h are possible in eastern Ontario, and possibly along the shorelines of Lakes Ontario and Erie overnight Sunday into Monday. Gusts could reach 100 km/h for extreme eastern Ontario, southern Quebec, the Maritimes and Gaspe Peninsula through the day Monday. Meanwhile, gusts could exceed 100 km/h for the Maritimes.

Power outages will be a big concern across these areas. Those with scheduled flights in or out of the northeastern U.S. on Sunday and into eastern Canada on Monday should monitor delays and cancellations.

With the storm being strongest the day before Halloween, forecasters suggest temporarily taking down or otherwise securing Halloween decorations.

Behind the storm, a sharp cooldown looms, with single-digit daytime highs expected, making for a chilly night for trick or treating.